1. Technical Field
The invention is a network of interconnected zones of intelligent nodes which employs a distributed protocol process performed by all nodes and a polling process performed by nodes which are master or substation units.
2. Background Art
Data communication in a topologically complex network of intelligent nodes is needed in many applications of current interest. As one of many possible examples, electrical power distribution systems could be operated more reliably or economically if there were more responsive and intelligent monitoring and control mechanisms throughout the system. It is a goal of the invention to provide a microprocessor-based intelligent monitoring and control device at each node in a communication network which is co-extensive with the electrical power distribution system and in which the nodal monitoring/control devices throughout the network communicate with computers in local electrical power distribution substations in the power distribution system. Preferably, various nodes in the network would be co-located with key locations, such as branch points, in the electrical power distribution system. However, it has not seemed possible to realize such a goal due to technical and economic constraints.
One technical constraint is arbitrating among several nodes attempting to communicate simultaneously so as to avoid conflicts among the nodes or collisions of messages from different nodes. This problem is particularly acute when all of the nodes are connected in parallel or share a common channel. One solution would be to individually connect all nodes to a central computer. However, this would be uneconomical and cumbersome for large power distribution systems.
For nodes connected in series, one solution might be to communicate with a given node from another location by sending a message containing an address list of the nodes through which the message is to be serially passed. This technique, however, is unreliable because if any one of the nodes on the list happens to be inoperative or if the link to that node is interrupted, the message cannot reach its destination.
Another solution is distributed protocol control. The known technique for this solution requires that all nodes be serially connected in a single ring. Thus, the technique is also susceptible to failure of a single node or interruption of any link between nodes. The technique is illustrated in FIG. 1, which is a block diagram of a token ring network in accordance with the prior art. Each node 10 is a computer which can communicate with any other node 10 in the network via a common communication channel 12. A unique token message circulates around the ring of nodes 10. The token message is recognized by each of the nodes 10. Upon receiving the token, a node wishing to Communicate replaces the token with a message on the channel 12. The token is then inserted behind the message, so that other nodes can access the bus. One of the nodes 10 is an active monitor which eventually removes the message to prevent its endless circulation in the ring.
A significant problem is that this simple distributed protocol control will not operate unless the network is a simple ring. Thus, nodal networks comprising multiple interconnected rings of nodes cannot use such distributed protocol control, as will now be described. Unfortunately, it is this very type of network which would be most useful in controlling many types of apparatus, such as electrical power distribution networks.